1. Field of the Invention
The present invention relates to a switching regulator used as power supply for an inverter circuit for driving a electric motor by boosting a low voltage (24 V, 48 V, etc.) of a battery mounted on an electric vehicle, as a power supply of a solar cell for high output, or for completely consuming an energy of a super capacitor or the like.
2. Description of the Related Art
Up to now, there is one type of switching regulators for obtaining an output voltage higher than that of an input voltage, which is made up of a boost chopper circuit, for example, as shown in FIGS. 7 and 8.
In FIG. 7, an inductance 2 is connected in series to a plus side of a D.C. power supply 1, an anode side of a diode 6 is connected to an output side of the inductance 2, and a cathode side of the diode 6 is connected to a node 50 at which the smoothing capacitor 14 and a load 16 are connected in parallel. Then, in order to hold a voltage across the load 16 constant, there is provided a control circuit 15 for controlling the on/off operation of a switching element (FET) 10 that short-circuits a node between the inductance 2 and the diode 6 and a minus side of the D.C. power supply 1.
The control circuit 15 operates so that in an on-state of the switching element (FET) 10, the load 16 is driven by a voltage VI of the D.C. power supply 1 while a magnetic energy is stored in the inductance 2. Then, when the switching element (FET) 10 becomes in an off-state, the magnetic energy stored in the inductance 2 and the voltage VI of the D.C. power supply 1 are superimposed on each other to drive the load 16.
FIG. 8 shows a circuit structure in which a circuit consisting of an inductance 26, a diode 30, a switching element (FET) 22 and a smoothing capacitor 34 which is identical in structure with the circuit of FIG. 7 is connected in series to the circuit of FIG. 7. In the figure, when the switching elements (FETs) 10 and 22 are in the on-state, the load 16 is driven by the voltage VI of the D.C. power supply 1 while a magnetic energy is stored in the inductances 2 and 26. Then, when the switching elements (FETs) 10 and 22 become in the off-state, the magnetic energy stored in the inductances 2 and 26 and the voltage VI of the D.C. power supply 1 are superimposed on each other to drive the load 16.
In both of the above two circuits shown in FIGS. 7 and 8, because a current I.sub.c1 or I.sub.c2 flowing in the capacitor 14 or 34 becomes a large triangular-wave ripple current, a current of several tens amperes or more flows in the capacitor 14 or 34 particularly when the load 16 is large in capacitance (1 kW or more), to thereby make the heating of the smoothing capacitors 14 and 34 high. As a result, there are disadvantageous in that not only the smooth capacitors must be made larger in size but also the lifetime becomes shorter.
Also, when the switching elements (FETs) 10 and 22 are in the on-state, that is, when the magnetic energy is stored in the inductances 2 and 26, the output voltage is equal in value to the input voltage VI. Therefore, a discharge energy from the smoothing capacitors is relied on in order to obtain a large output voltage, and large-scaled smoothing capacitors are required. This causes not only that the above disadvantages are made remarkable but also that in the case where the energy consumption of the load is particularly large, the magnetic energy stored in the inductances 2 and 26 is completely discharged, to thereby disable the load to be continuously driven. Thus, there is room for improvement.